Temperature indicators



2,928,791 TEMPERATURE INDICATORS Joseph D. Loconti, Natick, Mass, assignor to the United States of America as represented by the Secretary of the Army r No Drawing. Original application February 12, 1953,

Serial No. 336,652, new Patent No. 2,799,167, dated July 16, 1957. Divided and thisapplicatio'n June 20, 1957, Serial No. 667,055 a '7 Claims. Cl; 252 408) (Granted under Title 35, U.S. Code'(1952), see-266) The invention described herein, if patented, may be manufactured and usedby or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This application is a division of my copending application Serial 336,652, filed February 12, 1953, for Temperature Indicators, now Patent No. 2,799,167.

This invention relates to thermal indicators, of the type wherein a coating is applied to a base and irreversibly changes its appearanceat'a predetermined elevated temperature due to physical changes occurring therein at such temperature.

Devices wherein a coating is applied to a base, and

changes its appearance at elevated temperatures by reason of chemical changes, such as oxidation or decomposition, are'well known. It has also been proposed to aflix a white organic pigment to a dark base by means of a transparent resin and to observe the visual change occurring above the melting point of such pigment]. Both methods suffer from lack of precision, in that 'the oxidative or decomposition reaction is usually graduahand that the presence of a resin introduces a variable'which is 'inirhical to a ble; an example of such a binder and solvent'are, respec tively, a lower alkyl cellulose and water. Or, the same pigment is admixed with the same binder and solvent and is applied therefrom to a highly absorptive base, e.g., highly porous black paper. Both methods may be combined, e.g., by preparing an intimate mixture of a finely divided pigment, carbon black and binder, and applying them to a base which may but need not be highly porous;

as long as the pigment is in the crystalline state, the coat-- ing will appear gray, but will speedily turn black as soon as the pigment is melted and is absorbed in that state by E e the carbon black. 5

It is thus an objectof my invention to provide-a thermally indicating coating composition which accurately, rapidly and irreversibly shows that a certain predetermined temperature has been reached. 7

Another object of my invention is a series-of temperature'indicators which may be conveniently applied to a test object, and whose visual appearance at the termination of the test will indicate the temperature limit reached during the test. p y

A further object of my invention is a temperature indicative composition which; would firmly adhere to a porous or non-porous base by means of a binder which 2,928,791 Pistented- Mar. 15, 1960 2; will not interfere with "the accuracy of the temperature readings. A

Yet another object of my invention is a coated paper instantly and irreversibly responsive to a sharp but transitory upward temperature change, as, for instance, in telegraphy or modern high speed reproduction systems, as well as in the high speed recording of electronic computation data. 7

Still another object of my invention is a thermally indicating" composition operating by non-chemical visual changes, which is stable against accidental decomposition and photochemical deterioration.

A still further advantage of a composition of my invention is the ease with which it can be applied to a base by means of a brush, printing roller or other conventional coating implement.

With these and other objects and advantages in mind, I shall now set forth, by way of example, various 'erhbodiments of my invention and of the manner in which the same may be'practi'cedr It will be understood, of course, that such examples are by no means exhaustive of the many variation'sof which the practice of my invention is capable without departing from the spirit thereof.

Inasmuch as the pigments of my'invention are crystalline, they will not adhere to the backing surface without a binder. I therefore prepare a coating composition for application to the backing by grinding the crystalline substances together with a suitable binder, e.g., polyvinyl alcohol or a lower alkylor alkoxy alkyl cellulose ether such as for instance, methyl cellulose, and a solvent in which said binder is soluble but said crystalline compound is non-soluble (e.g., water in the case of water-insoluble pigments), until acoatable slurry is obtained, and then apply the coating composition to the backing by means of a brush, printing roller or similar conventional means; other soluble binders are, e.g., pectins, hemicellulose, gelatin, soluble starch; By wayQof example, such a coating composition may be composed of about 5 to about 50 parts of crystalline pigment, about 1 to about 12 parts of binder-(e.g., methyl cellulose or polyvinyl alcohol) and enough waterto give parts of composition. The thickness of the coatings preferably range between about .00005 and about .0025"; thicker and thinner coatings may-also be employed,'but it should be borne in'mind 5 that too thick a coating may'not satijsfac'torily absorb into the inner region of the absorbent surface, and that too thin a coating maybe are transparent and too likely-to rub on? to be of much practical use. 3 As an alternate method, a top coating of acompatible transparent substance may be employed to fix the'deposit in place; this top coating may be of the same material as the above-described binders, e.g., polyvinyl alcohol or methyl cellulose. lnthis alternate method, an aqueous dispersion of the pigment (e.g. 25% solids conten'tlis coated on the backing, and the transparent top coat of binder material is thenapplied.

I compound a crystalline compound having a sharply defined melting point with a finely divided absorbent sub- "stance, e.g., carbon black. In the case of a white pigso ment, this composition will appear gray. By addition of suitable binder, this composition may be coated directly on'thc object to be tested. When the melting point of the pigment is exceeded, it liquefies and is absorbed by the carbon black, thereby changing the visual appearance of the coating to black. Instead of carbon black, another finely divided black substance, e.g., black iron oxide, copper oxide, copper sulfide, or lead sulfide may be used, ora coloredmeltable pigment may becompounded with a white powdered substance and binder; a'ftersuch composition is applied tothe test object, and the melting point of thepigment is exceeded, the latter will melt and'difiuse into the white carrier. Binders suitable for this modification of my invention are those which are soluble in a liquid in which the crystalline compound is not soluble, e.g., methyl cellulose andwater, respectively. Instead of being directly coated on the test object, these compositions may also be coated on the backing of an absorbent or non-absorbent surface, e.g., pressure-sensitive cellulose adhesive tape, which may then be subdivided into desired lengths and applied to the test object. Other suitable carriers would be metal, glass, etc.

I have worked out a catalog of white crystalline waterinsoluble pigments having sharply defined melting points within the limits of to 304 C., and therefore suitable for the purposes of my invention; this list is reproduced below in Table I:

Table l Melting Transition Pigment Point, Temper- C. ature, 0.

triortho-cresyl thiophosphate 45-46 45 triphenyl phosphate 49-50 47 dl-(n-tetradecyl sebacate) 50 48 palmitic acid 57-59 n-octadecyl alcohol 56-57 56 tripalmitin 63 5-65 61 benzhydrol 61 4,4-dibromodiphenyl ether 62 di-n-butylthiourea 62 stearic acid 68 d-di-n-butyl 71 5-73 68 dicetyl -70 69 4-chlor0be'1zop 66-70 74 tri-p-cresyl phosphate. 75. 5-77 74 p-ethoxydiphenyl. 71-72 74 acetoacetenilide 84-85 75 sucrose octa-acetate 79 n-butyl oxamate 86-87 83 m-dipheuyllemene. 85-86 84 benzotriazole 94-94. 5 86 tril: enzylamire 91-92 92 triphenylntethare 91. 5-93 92 hydroxyhydroquinone triacetate. 93-94 94 pimelic acid 103-305 94 2-morcaptothiazoline 105-106 2,4,6-trinitro-1.3-dirnethyl-5-tert. butylbenzene 112-113. 5 102 m-toluic acid 109-112 103 2,4dibromo-naphthol 108-109 105 dichlorodipheuyltrichloroethane 110. 6 107 o-benzoyltenzoic acid 127-128. 5 110 diacetyl hydroquinone 119 118 benzoic acid 121. 7 118 127-128 121 phenylthieurea 150 5-151 148 m-clilorbenzoic acid. 152 5-153 150 sultanilamide 163-164 156 beuzalacetophenone dibromide. 156-157 157 triphenylcarbinol 161-162 160 pyrogallol triacetate.-. 163-164 161 benzanilide 162-163 161 diphenylpiperazine 165-166 164 hydroquinone.-. 171-173 164 thiourea 173-175 167 m-aminohenzoic acid.- 172-173 169 diamyl hydroquinone. 179-181 170 b-methylanthraquinone- 172-174 172 s-di-o-tolylthiourea- 186 175 anisic acid 181-184 177 d-camphoric ac 186-187 178 succinic acid. 183-184 180 p-arniuobenzoic a 186-187 182 hippuric acid 187-189 184 p-hydroxybenzoic acid. 214-216 200 dicyandiamidc 209-210 205 2,5-di-tert. butyl hydroquinone. 213-216 209 p-diphenylbeuzene 212-213 209 saccharine 224-228 220 4,8-dimethyl-Z-hydroxyquinoline. 223-224 217 carbanilide 241-242 240 nlmnnlnhthalein 260-261 257 theobromine 2 330 9 304 Temperature at which the indicator just turns black when placed or 10 seconds between two heated aluminum blocks.

1 In sealed tube.

It will be noted from the last column of the above table that the transition temperature, i.e., the temperature at which the visual appearance of the thermal indicator actually changes, is in most instances 1 or 2 C. below the melting point. While several theoretical explanations for this phenomenon are available, I do not wish to be bound by any of them, but merely state that this phenomenon is as readily observable as the transition of the compound from its solid to its liquid state, and can be predetermined for each given compound. I therefore use the terms melting point and transition temperature interchangeably in the specification and claims.

An example of a colored pigment capable of the same use is dimethylamino azobenzene, melting point 116-117" C., transition temperature 114 C. A mixture of 15 parts of dimethyl amino azobenzene, 45 parts of titanium dioxide, 2.5 parts of 15 C.P. methyl cellulose and 175 parts of water appears yellow at room temperatures; above 114 C. the pigment diffuses and thereby turns the color of the composition to orange.

Other specific examples of temperature-indicating coating compositions in accordance with my invention are:

(A) 25 parts of stearic acid, 1.25 parts of water soluble methyl cellulose and 74 parts of water are milled in a conventional pebble mill for 24 hours. The resulting white dispersion is sprayed on a porous black surface in a thin even coat. Transition temperature: 68 C.

(B) White non-absorbing paper is coated with a mixture of 10% Carbon Black G and 1% 400 C.P. Methocel in water. This base coating is then overcoated with a coating composition of a white crystalline pigment and methyl cellulose. With triphenyl phosphate as the pigment, transition temperature was observed to be 47 C., with stearic acid 68 C., hydroquinone dibenzyl ether 127 C., benzanilide 161 C., dicyandiamide 204 C.

(C) A mixture of 20 parts triphenyl phosphate, 1.8 parts carbon black (Micronex, Columbian Carbon Company, New York), 2 parts 15 C.P. Methocel methyl cellulose, and 76.2 parts water is coated or printed on a metallic surface. At room temperature, the coating appears gray; upon exceeding the transition temperature of 47 C., the space occupied by the coating appears black.

(D) 25 parts of di-n-butyl thiourea are ground to a slurry with .6 part of Mapico Black (black iron oxide manufactured by Columbian Carbon Company), 1.25 parts of 15 C.P. methyl cellulose and 7315 parts of water. The composition, which has a gray color is sprayed on a nonabsorbent white backing; at 61 C. it turns black.

' of uses and applications. able number of variations and specific examples of my In yet another variation of my invention, I have found that a monomeric polymerizable crystalline compound whose melting point substantially coincides with the temperature at which polymerization takes place, can be used in a temperature indicator. If such a compound is once permitted to melt and polymerize upon exposure to the critical elevated temperature, subsequent lowering of the temperature below the melting point of the compound will nevertheless not result in depolymerization. Consequently, it will not be necessary to employ a highly absorbent backing with a pigment of this type inasmuch as the irreversible change in visual appearance is independent of any absorption in the surface of the backing. An example of such a pigment is sucrose-octa-acetate which melts and polymerizes at 85 and has a transition inexpensive temperature indicators, or thermally indicative direct coatings, which are capable of a large variety Having described a considerinvention, I wish to be understood that they are illustrative in nature and may be departed from, while still remaining the spirit and scope of my invention.

having a melting point or thermal decomposition point a substantially higher than the melting point of said organic compound and non-reactive with said organic compound in the solid phase; whereby when said mixture is exposed to a temperature substantially corresponding to the melting point of said organic compound, said organic compound melts and is absorbed by said carrier, which results in a readily observable irreversible color change of said composition to black.

an upward change in thermal conditions, comprising an intimate mixture of a finely divided white crystalline organic compound having a sharply defined melting point 2. A composition of matter according to claim 1, V

wherein said finely divided Solid carrier is carbon black.

3. A composition of matter for irreversibly indicating an upward change in thermal conditions, comprising an intimate mixture of a finely .divided substantially waterinsoluble colored crystalline organic compound having a sharply defined melting point of at least 45 C., an inorganic pigment of a contrasting color as a carrier therefor, said inorganic pigment being non-reactive with said organic compound in the solid phase, an uncolored water-soluble binder, and enough water to produce a slurry, said slurry beingground together to form a colored dispersion adapted for application as a coating, whereby when said coatingis exposed to a temperature substantially corresponding to the melting point of said organic compound, said organic compound melts and is absorbed by said carrier, which results in a readily observable irreversible change of the appearance thereof.

4. A composition of matter for irreversibly indicating an upward change in thermal conditions, comprising an intimate mixture of a finely divided substantially waterinsoluble colored crystalline organic compound having a sharply defined melting point of at least 45 C., a

white inorganic pigment as a carrier therefor, said in-' organic pigment being non-reactive with said organic compound in the solid phase, an uncolored water-soluble binder, and enough water to produce a slurry, said slurry being ground together to form a colored dispersion adapted for application as a coating, whereby when said coating is exposed to a temperature substantially correspending to the melting point of said organic compound,

of at least 45 C., carbon black, an uncolored watersoluble binder, and enough water to produce a slurry, said slurry being ground together to form a gray dispersion adapted for application as a coating; whereby when said coating is exposed to a temperature substantially corresponding to the melting point of said organic compound, said organic compound melts and is absorbed by said carbon black, which results in a readily observable irreversible change of the appearance of said coating.

6. A composition of matter for irreversibly indicating an upward change in thermal conditions, comprising an intimate mixture of a finely divided monomeric polymerizable crystalline organic compound which melts and polymerizes at a sharply defined predetermined temperature of at least 45 C., and of a finely divided solid carrier of acontrasting color and having a melting point or thermal decomposition point substantially higher than the melting and polymerization point of said compound and non-reactive with said organic compound in the solid phase; whereby when said mixture is exposed to a temperature substantially corresponding to the melting and polymerization point of said compound, said compound melts and polymerizes and is absorbed by said carrier, which results in a readily observable irreversible color change of said composition.

7. A composition according to claim 6, wherein said compound-is sucrose octa-acetate.

References Cited in the file of this patent UNITED STATES PATENTS 1,844,199 Bicknell Feb. 9, 1932 1,924,793 Laske Aug. 29, 1933 2,606,654 Davis Aug. 12, 1952 2,649,382 Vasce Aug. 18, 1953 2,799,167 'Loconti July 16, 1957 OTHER REFERENCES Colour index, 2nd edition, volume 2, p. 2707, pub. 1956 by the Society of Dyers and Colourists of Bradford, England. 

3. A COMPOSITION OF MATTER FOR IRREVERSIBLY INDICATING IN UPWARD CHANGE IN THERMAL CONDITIONS, COMPRISING AN INTIMATE MIXTURE OF A FINELY DIVIDED SUBSTANTIALLY WATERSOLUBLE COLORED CRYSTALLINE ORGANIC COMPOUND HAVING A SHARPLY DEFINED MELTING POINT OF AT LEAST 45*C., AN INORGANIC PIGMENT OF A CONTRASTING COLOR AS A CARRIER THEREFOR, SAID INORGANIC PIGMENT BEING NON-REACTIVE WITH SAID ORGANIC COMPOUND IN THE SOLID PHASE, AN UNCOLORED WATER-SOLUBLE BINDER, AND ENOUGH WATER TO PRODUCE A SLURRY, SAID SLURRY BEING GROUND TOGETHER TO FORM A COLORED DISPERSION ADAPTED FOR APPLICATION AS A COATING, WHEREBY WHEN SAID COATING IS EXPOASED TO A TEMPERATURE SUBSTANTIALLY CORRESPONDING TO THE MELTING POING OF SAID ORGANIC COMPOUND, SAID ORGANIC COMPOUND MELTS AID IS ABSORBED BY SAID CARRIER, WHICH RESULTS IN A READILY OBSERVABLE IRREVERSIBLE CHANGE OF THE APPEARANCE THEREOF. 